Projection of laser beams requires an aperture diameter, or effective aperture diameter, large enough such that the diffraction limit of the optical system, given by λL/D (where λ is the wavelength of light, L is the propagation distance, and D is the aperture diameter), is sufficiently small to provide the required beam size on the target of interest. By the same token, imaging requires that the aperture diameter be large enough such that the diffraction limit is smaller than the object features to be resolved by the imaging system. The size and cost of conventional telescope systems, which utilize a large monolithic mirror, or several large mirror segments, to achieve these requirements can be exorbitant. An alternative technology that has been pursued for many years is that of phased array beam combining and phased array imaging. In a phased array system, a plurality of smaller telescopes are used in a coherent fashion so that the plurality of smaller telescopes, when combined coherently, have the same diffraction limit as a larger conventional telescope.
While this appears on the surface to offer a creative and viable solution to the size and weight problems associated with large aperture optical systems, there is a significant lack of insight and methods for means to actually achieve coherent phasing of the array. While much of the prior art focuses on creative devices for phased array beam control and/or imaging (see references: 1, 2, 3, 4, 5), only a small fraction of prior art focuses on the sensing and control required for active phasing of the array (see references 6, 7, 8, 9). The published methods for beam phasing and imaging utilize hill climbing metric optimization based methods whose performance is heavily dependent on the choice of metric and are usually inherently limited in the speed at which compensation can be achieved.
This is particularly true if the phased array must operate in a real time manner for compensation of the effects of turbulence on laser propagation. In the case of laser propagation, the prior art relies on a closed loop feedback signal that operates over the round trip from the transmitter to the target and back, inherently limiting the characteristic compensation time constant to be at least roughly 20 times slower than the round trip time of flight to the target and back. While this characteristic compensation time may be acceptable for some applications, it is inadequate for the majority of applications and is certainly not adequate for compensation of phasing errors resulting from on board vibration sources. On board vibration sources can be the most significant source of phasing errors—particularly if fiber laser and amplifier systems are used, for which phase aberrations are inherently highly susceptible to both mechanical and acoustic vibration sources. The present invention is relevant to general phased array systems, but in particular focuses on the challenging problems posed by phased arrays of fiber laser systems.
What is needed is a method for coherently combining a plurality of subapertures to phase a plurality of beams at a target. The method must not require a monolithic beam director (which would eliminate the size and weight advantages of the phased array). The method must be capable of being effective with moving targets and platforms. The method must have a means to accomplish isolation of the transmitted beam from a return beam used for measurement of aberrations in the path. The method must not rely on measurements from one wavelength of light to compensate through fiber optical transport at another wavelength of light (this is due to the fact that measurements through fiber can only be measured within a single wavelength—unless an ultra-precise fiber length measurement system is incorporated into the system, in which case such a system must be integrated without a non-common path that passes through fiber optic beam transport). The method must utilize a common reference plane to serve as the “zero phase reference” for the phased array. The method must not have any non-common path that is in fiber optical transport. The method must accommodate a means for on-platform stabilization of high speed, large amplitude aberrations that occur in either a fiber amplifier or due to mechanical/acoustical vibrations. The method must have a means for measurement of the tilt error on each subaperture and must have a means for measurement of the global tilt error of the entire array.
The present invention provides for a means to meet these requirements in an innovative fashion that enables the potential size and weight advantages of coherent phased array laser and imaging systems to be realized.